In recent years, screening for weapons at entrances of public places, such as airports, government buildings, public schools, and amusement parks, has increased to ensure safety for the public at those places. Screening for weapons can include requiring people entering such public places to pass through a magnetic screening system such as a portal metal detector. Although people have become accustomed to passing through portal metal detectors, the process remains relatively slow for a number of reasons.
One reason for the slow process is that people must empty their pockets of all metallic objects, remove their coats, and sometimes remove their shoes. The objects and clothing are either physically inspected by hand or passed through an x-ray machine for visual screening. Another reason for the slow process is due to false detection or detection of non-weapon metals, such as wrist watches, belt buckles, metallic wires found in ordinary garments, personal adornments such as broaches or hair clips, and loose coins in pockets.
Yet another reason for the slow processing is due to machine settling times, which refers to the amount of time that must be allowed for the sensors in metal detectors to resettle after a person passes through it. Allowing a portal metal detector sufficient time to settle ensures accurate readings of the next person. A person who passes through the portal metal detector then stops on the other side very close to the portal metal detector (i.e., within an ‘influence’ zone) can also influence the metal detector to such an extent that the metal detector makes a false detection or misses detecting an object as the next person passes through it. Therefore, portal metal detector operators must stop the next person from entering the portal metal detector until the previous person has passed beyond the influence zone.
Typically, all of the delays result in a passthrough rate of between one and two hundred persons per hour. To accommodate large crowds, many portal metal detectors are operated in parallel, which leads to staffing, training, and machine calibration issues. If the passthrough rate were higher, many venues that are currently equipped with large numbers of portal metal detectors could reduce the number in use, and venues such as sports stadiums not currently equipped with portal metal detectors would be so equipped.
Moreover, today's portal metal detectors are sensitive to large ferromagnetic objects, such as wheelchairs. When a person in a wheelchair passes through the portal metal detector, the portal metal detector is overwhelmed by the metal content of the wheelchair and unable to detect relatively small metal objects on the person. In addition, even if the wheelchair is not passing through the portal metal detector, it can influence the detector to such an extent that the detector makes erroneous readings.
In addition to the slow process associated with today's portal metal detectors and their sensitivity to large ferromagnetic objects, many portal metal detectors are ‘active,’ meaning they emit an electromagnetic field in a volume of interest (i.e., the area in the portal metal detector). Active detectors can be dangerous for people using medical devices, such as pacemakers, that are sensitive to electromagnetic fields. Passive metal detectors, which sense a local disturbance in the earth's magnetic field, do not affect medical devices, but they are sensitive to local magnetic fields, large ferromagnetic devices, calibration errors, background offsets, and other measurement disturbances known in the art.